Handle builds without gearman jobs

Zuul creates fake Build objects for jobs that it skips due to
merge conflicts and other such situations.  If Zuul decides to
cancel such a build, we should handle it gracefully instead of
throwing an exception.

Also log received gearman job handles to better correlate builds
with gearman debug logs.

Change-Id: I1cd14c8249ac4c68e797430b2b4941f2eeee2acf

zuul/launcher/gearman.py

@@ -321,12 +321,21 @@ class Gearman(object):
                            gearman_job)
             self.onBuildCompleted(gearman_job, 'NO_HANDLE')
 
+        self.log.debug("Received handle %s for %s" % (gearman_job.handle,
+                                                      build))
+
         return build
 
     def cancel(self, build):
         self.log.info("Cancel build %s for job %s" % (build, build.job))
 
         build.canceled = True
+        try:
+            job = build.__gearman_job  # noqa
+        except AttributeError:
+            self.log.debug("Build %s has no associated gearman job" % build)
+            return
+
         if build.number is not None:
             self.log.debug("Build %s has already started" % build)
             self.cancelRunningBuild(build)